Subcarrier multiplexing (SCM) of an optical carrier has been proposed as an alternative to time division multiplexing in multiple access lightwave systems (T.E. Darcie, Subcarrier Multiplexing for Multiple Access Lightwave Networks, J. Lightwave Technology, 1987, LT-5, pp. 1103-1110). In a SCM scheme, an optical carrier is intensity modulated by one or more subcarriers at microwave frequencies, each of which provides an information channel.
A typical receiver configuration for a SCM system is shown in FIG. 1. A photodetector 10 (typically a pin diode) detects all the subcarrier channels, but only the required channel need be demodulated. The receiver sensitivity is determined by the detector electronic bandwidth and the bandwidth of individual channels. A microwave preamplifier 11 amplifies the subcarrier channel of interest prior to downconversion in a mixer 12 fed by a local oscillator 13. A disadvantage with this configuration is that the noise figure of available preamplifiers 11 increases as the operating frequency increases, and the receiver sensitivity decreases correspondingly.
An alternative detection technique has been proposed (T.E. Darcie et al., Optical Mixer Preamplifier for Lightwave Subcarrier Systems, Electronic Letters, 1988, Vol. 24, pp. 179-180), in which the subcarrier signals are downconverted optically by an optical mixer 15, before photodetection. See FIG. 2. The downconverted optical signals are then detected using a low speed photodetector 16 and preamplifier 17. In this proposed configuration, the optical mixer is constituted by a semiconductor optical amplifier 15, the gain of which is modulated by applying modulated electrical bias current to the amplifier 15 from a local oscillator 18. An incoming intensity modulated (IM) optical signal propagating through the amplifier interacts with the modulated gain to produce frequency shifted components of the intensity modulation signal. The optical mixer thus depends on the fact that the gain of the semiconductor optical amplifier used is dependent on the bias current applied. However, a drawback of this arrangement is that the maximum frequency at which gain modulation can be achieved is limited by the carrier lifetime of the semiconductor optical amplifier 15, as is illustrated by line A of the gain-modulation/modulation-frequency graph shown in FIG. 4.
Accordingly, an object of the present invention is to provide an optical mixer capable of operating at frequencies well into the microwave range. A further object of the invention is to provide such an optical mixer in an optical receiving system, thereby eliminating the need for high frequency preamplifiers and there associated high noise figures. A still further object of the present invention is to provide a high sensitivity optical receiver. The present invention achieves these goals.